1. Field of the Invention
The present invention relates to a circuit for controlling a DC/DC converter and its control circuit, and more specifically to a technology of improving the efficiency of the DC/DC converter.
2. Description of the Related Art
A DC/DC converter converts an input DC voltage into different DC voltages for output in various uses. Actually, it is applied to various uses. The output voltage should be stable at a predetermined value and the power consumption of the converter should be smallest possible in consideration of the case where the converter is provided for a portable terminal unit.
FIG. 1 shows the configuration of an example of the conventional DC/DC converter. The DC/DC converter shown in FIG. 1 converts an input voltage V.sub.in into an output voltage V.sub.out.
A pulse frequency modulator 101 receives the output voltage V.sub.out as a feedback signal, and outputs a pulse signal which is kept ON for a predetermined time at a frequency based on the output voltage. A driver 102 outputs an ON/OFF control signal according to the pulse signal received from the pulse frequency modulator 101. A switching element 103 functions as switch according to the ON/OFF control signal output by the driver 102. The switching element 103 is, for example, a P-channel MOS transistor, and is turned on when the output from the driver 102 is at an L level.
When the switching element 103 is turned on, the current passing through the switching element 103 flows through coil L, thereby providing the electric power for the capacitor C and the output terminal and raising the output voltage V.sub.out. As a result, when the output voltage V.sub.out drops, the pulse frequency modulator 101 outputs a control signal to set the switching element 103 in an ON state and keeps the output voltage V.sub.out constant.
FIG. 2 shows the output of the pulse frequency modulator 101. In this example, the pulse signal (the period in which the H level is entered) sets the switching element 103 in an ON state.
When the load connected to the output of the DC/DC converter is small, the output voltage V.sub.out can be held at a reference value even if the current provided for the load is small. Therefore, the pulse frequency modulator 101 outputs pulse signals at a relatively low frequency as shown in FIG. 2A. When the load becomes large, it is necessary to provide a large current for the load so that the output voltage V.sub.out can be held at a reference value, and therefore the pulse frequency modulator 101 increases the frequency of the ON state of the switching element 103 with the pulse frequency kept high as shown in FIG. 2B. As described above, the DC/DC converter keeps the output voltage V.sub.out at a constant value.
However, when the DC/DC converter is used for, for example, a portable terminal unit, etc., the power consumption should be reduced to the lowest possible level to prolong the operation time (durability of the battery) of the terminal unit. However, the above described conventional DC/DC converter has not been successfully developed as a power saving unit.
That is, considering the DC/DC converter loaded onto a portable terminal unit, etc., the DC/DC converter is necessarily small. In this case, the above described capacitor C and coil L should be small. To make the capacitor C and coil L small, it is necessary to raise the frequency of an oscillator (built in the pulse frequency modulator 101 as shown in FIG. 1) for generating pulse signals to control the switching element 103 as high as several hundred KHz.
Thus, if the frequency of the operation of an oscillator is raised, the power provided at a single pulse is reduced, thereby increasing the number of pulses required to provide a predetermined amount of power for the load. Since the pulse refers to a signal to turns on or off the switching element 103, the switching frequency of the switching element 103 is necessarily raised when the frequency of the operation of the oscillator is raised. Since a loss accompanies the operation of the switching element 103, the conversion efficiency is lowered with the increasing frequency of a switching operation through the switching element 103.
With the above described configuration, an oscillator is required (built in the pulse frequency modulator 101). Since the oscillator is constantly in the operating state, it also prevents the successful reduction of the power consumption. Furthermore, if the frequency of a pulse is altered as described above, the ripple of the output voltage V.sub.out becomes inconstant.
Thus, the conventional DC/DC converter has not been developed as a power saving unit.